This application claims the priority of German Patent Application, Serial No. 101 41 351.3, filed Aug. 23, 2001, pursuant to 35 U.S.C. 119(a)-(d), the subject matter of which is incorporated herein by reference.
The present invention relates, in general, to a hydraulic system, and in particular to a hydraulic system for providing a fluid under pressure to consumers of an injection molding machine.
German patent publication DE 196 21 907 A1 describes a hydraulic system, in particular for an injection molding machine, including two variable displacement pumps which are each driven via respective couplings by an electric motor. The output sides of the displacement pumps can be interconnected by a valve. Each displacement pump is fluidly connected by a pressure line to a number of consumers via valves. When the pressure lines are not connected together, their pressure is normally different. This conventional hydraulic system suffers shortcomings because the supply of consumers, connected to the same displacement pump, is no longer assured, when a pressure drop is encountered in one of those consumers. In the event the pressure lines of the displacement pumps are connected together, the supply to all consumers of the hydraulic system can then no longer be assured.
It would therefore be desirable and advantageous to provide an improved hydraulic system to obviate prior art shortcomings and to ensure a supply of consumers, even if a breakdown in one of the consumers is encountered.
According to one aspect of the present invention, a hydraulic system, in particular for an injection molding machine, includes at least two consumers, at least two displacement pumps, at least one first control valve for each of the consumers, and at least one second valve for controlling a fluid communication between the outlet sides of the displacement pumps, wherein at least one of the consumers can be supplied with pressure from the outlet side of the displacement pumps separately or together in groups.
The present invention resolves prior art problems by supplying a consumer with fluid under pressure from the output side of at least two displacement pumps, preferably via a 2/2 directional control valve, whereby the pressure to the consumer from the outlet of the displacement pumps can be supplied separately or together. When a pressure drop is encountered in the pressure line of one of the displacement pumps, the other pressure line of another displacement pump is still under pressure. In the event, two or more pressure lines are connected together, the pressure line encountering a pressure drop is simply cut.
Suitably, the hydraulic system has two displacement pumps, wherein the output pressure of the displacement pumps is supplied to each of two consumers via two valves. Of course, the hydraulic system may have more than two displacement pumps whose output pressure can be supplied to all consumers. When using a hydraulic system in combination with an injection molding machine, costs and economical considerations dictate the use of two consumersxe2x80x94closing unit and injection unitxe2x80x94which are each supplied with the output pressure of two displacement pumps via two 2/2 directional control valves. In the event both 2/2 directional control valves are opened, the pressure on the output side of both displacement pumps is equalized. The displacement pumps are thus effectively connected together.
A hydraulic system according to the present invention has the advantage of a flexible assignment of resources to react to fluctuations in the load distribution. The demand on hydraulic fluid by a closing unit of an injection molding machine as consumer varies, for example, depending on the utilized injection molding tool. Depending on the need of hydraulic fluid under pressure, a displacement pump can be used which offers a pump capacity that suits best the hydraulic demand at hand. Thus, a displacement pump with small pump capacity would be used for a hydraulic cylinder with low hydraulic fluid volume to realize a better control dynamics. When a hydraulic cylinder with great hydraulic fluid volume is involved, both displacement pumps are connected together to reduce the filling time of the hydraulic cylinder and thus the cycle time of the consumer. Depending on the requirements, the operator of the injection molding machine is thus in a position to select an energetically beneficial parallel operation of several consumers or an increase in the operating speed through interconnection of the displacement pumps.
According to another feature of the present invention, the displacement pumps may be configured as electro-hydraulic variable capacity pumps. Hereby, the output pressure of the capacity pump is registered by a pressure sensor and converted into a voltage signal. The voltage signal represents the input variable of the control as an actual value. Computer-controlled programs that can be individually suited to the application at hand can be used to control electro-hydraulic capacity pumps.
According to another feature of the present invention, each of the displacement pumps generates an output variable, with the output variable of one of the displacement pumps representing an actual value which is used as a desired value for the output variable of the other one of the displacement pumps. As a result, both displacement pumps operate that in the so-called xe2x80x9cmaster-slave modexe2x80x9d. This type of control has the advantage that the displacement pumps are prevented from mutually building up one another or operate against one another.
According to another feature of the present invention, each of the capacity pumps may be constructed separately for pressure control and fluid flow control.
According to another feature of the present invention, there may be provided a pressure reservoir which is operatively connected to at least one of the consumers. In this way, the displacement pumps can be made smaller in dimension while still temporarily maintaining a high operating speed. Suitably, the pressure reservoir is operatively connected to the injection unit of an injection molding machine, having a high power consumption.
According to another feature of the present invention, there may be provided a drive unit, including an electric motor, for operating the displacement pumps, wherein one displacement pump is positioned on one side of the electric motor, and the other displacement pump is positioned on the other side of the electric motor. In this way, the displacement pumps can be linked by couplings in close distance to the environmentally-friendly drive unit.
According to another feature of the present invention, each of the displacement pumps may generate an output variable commensurate with a desired value which is limited by a maximum driving power of the electric motor. Such a restriction is possible because the power parameters of the individual displacement pumps are known at all times by ascertaining pressure and flow rate. Overloading of the drive unit is hereby avoided.